Background: Differentiating brain abscesses from necrotic cystic tumors is critical for clinical management but remains challenging due to overlapping features on conventional MRI. Aim: To evaluate the diagnostic performance of diffusion-weighted and morphological MRI biomarkers in differentiating brain abscesses from necrotic cystic brain tumors. Materials and Methods: This retrospective study included 54 patients with ring-enhancing brain lesions (brain abscess, n=13; metastasis, n=18; glioblastoma, n=23). MRI protocols (T1W, T2W, DWI, SWI, and T1+CE) were analyzed for lesion morphology, diffusion characteristics, capsule thickness, and the “dual rim sign.” Group comparisons were performed using ANOVA, chi-square, and Fisher’s exact tests. Diagnostic performance and optimal thresholds were determined using ROC curve analysis (p<0.05). Results: 1. Comparison of Diffusion Parameters: The core ADC ratio min was significantly lower in the brain abscess group than in the cystic-necrotic tumor group (0.812±0.228 vs 1.902±0.535 for glioblastoma, 1.590±0.394 for metastasis; p<0.001). The area under the curve (AUC) for core ADC ratio min was 0.977, indicating excellent diagnostic performance. 2. Capsule Characteristics: On SWI, the “dual rim sign” was detected in 92.3% of brain abscesses but was absent (0%) in all cystic-necrotic tumors (p<0.001). The abscess capsules were significantly thinner (mean 3.3 mm) and more uniform compared to the irregular, thicker walls of necrotic tumors. 3. Diagnostic Thresholds: The optimal cut-off value for the core ADC ratio min was 0.94 (sensitivity 92.3%, specificity 92.7%). For capsule thickness, the cut-off value was 3.3 mm with an AUC of 0.955. Conclusion Core ADC ratio and capsule thickness are reliable quantitative MRI biomarkers for differentiating brain abscesses from cystic-necrotic tumors. Integrating diffusion metrics with susceptibility-based and morphological features, such as the “dual rim sign,” significantly enhances diagnostic accuracy in clinical practice, facilitating non-invasive and timely diagnosis.